Definition

Anaphylaxis not explained by a proved or presumptive cause or stimulus; accounts for ~30-60% of cases of anaphylaxis in adults and ~10% of cases in children
It becomes a diagnosis of exclusion after other causes have been considered, such as foods, medications, exercise, food and exercise, insect stings or bites, mastocytosis, C1 esterase inhibitor deficiency/dysfunction, psychogenic reactions, and Munchausen anaphylaxis (intentional surreptitious ingestion of a known allergen)
- May be followed by panic attacks that can be confused with anaphylaxis
- May coexist with food allergy, exercise-induced anaphylaxis, chronic urticaria, food allergy
- Fatalities have been reported

Initial Approach

Discontinue medications that may complicate treatment or worsen an event:
- Beta-blockers
- ACE-inhibitors and ARBs
- MAO-inhibitors
- Certain tricyclic antidepressants (e.g. amitryptiline)
Encourage carrying an epinephrine auto-injector and wearing identification jewelry

Initial Treatment

Refractory Treatment

Epinephrine IM
Cetirizine or other H1 antihistamine PO
Albuterol 2-4 mg PO BID (if tolerated)
Prednisone PO if episodes severe or frequent
- 40-60 mg PO QD x 1-2 wk, then QOD x 1 mo, then taper by 5-10 mg monthly if the patient is not experiencing anaphylaxis
- Lack of response to prednisone (e.g. at least 40 mg PO QD for 2 weeks) suggests USIA subtype (see above), and treatment often paradoxically worsens this subtype

Note:

Vast majority of patients with IA gradually improve, including patients who have frequent episodes and require prednisone for months or even 2-3 years